1996 Pontiac Firebird misfires

I have misfire codes. I have changed plugs and wires, replaced all three ignition coils as one tested bad, had the ecm tested (good). I took to my mechanic and one fuel injector was bad and replaced. Wires and plugs were double checked. Fuel pressure was checked. I had a new fuel pump put in a month ago and it was running fine over a month. It is running 1000% better after injector replaced but is still throwing intermittent misfires on the cylinder with the new injector and the cylinder directly opposite, #6 and #3 I believe. The car has alot of miles but I believe the engine is strong internally as it still gets 22-24 mpg. I can't afford to continue pay for trouble shooting and am desperate. Any ideas?